Intercommunication system



Jilly 28, 1,964 .1. E. CHAPMAN 3,142,725

INTERCODNUNICATION SYSTEM Filed April 16, 1962 awn/Ton. "j 54' aonu EARLCHAPMAN "F153 MBMIZMK ATTORNEY United States Patent 3,142,725WTERCOMIVEUNICATION SYSTEM John Earl Chapman, St. Queries, 111.,assignor to Dukane Corporation, St. Charles, Ili., a corporation ofDelaware Filed Apr. 16, 1962, Ser. No. 187,815 4- Clairns. (Cl. 1791)This invention relates to an intercominunication (intercom) system andmore particularly to means for indicating the busy condition of astation in such a system. The present invention is limited to intercomsystems wherein at least three stations, two of which are masters, aremetallically interconnected by the system circuitry.

In the intercom art, it is well known that a so-called master station isa station which is associated with an audio frequency amplifiercooperating with a transducer which can function selectively as aspeaker or transmitter, the master station also including suitableswitching means for selecting a called station and also for determiningthe direction of transmission. While an intercom system embodying thepresent invention may include so-called remote stations (these simplyconsisting of a transducer), the present invention relies upon the factthat a busy remote station has connected thereto some other but busymaster station.

In its more general aspects, the invention takes advantage of themarginal conductive characteristics of a gaseous conduction lampgenerally designated as a neon light. Such lights contain spacedelectrodes within a glass envelope and operating in an environmentusually consisting of a rare gas such as neon, as one example. Suchlamps break down and become conductive above predetermined voltage,which voltage may depend upon the particular type of lamp involved. Oncesuch a neon lamp breaks down and has a gaseous discharge therein, thelamp will maintain the discharge providing the potential across the lampterminals remains above a minimum value, usually somewhat below theinitial breakdown potential. Such lamps exhibit a negative resistancecharacteristic and require conventional resistors as ballasts to permitthe lamp to operate satisfactorily. The current required for maintaininga discharge in some of the small neon lamps now available may bemeasured in microamperes. Thus, as an example, small neon pilot lightsavailable in the market will operate on potentials of the order of about60 volts and currents of the order of about 30 microamperes.

While a system embodying the present invention can use electromagneticdevices such as relays, the cost of a relay having even good sensitivityis far greater than the cost of a simple neon lamp. At that, such arelay would require far more current than a neon lamp. While extremelysensitive relays could be used with currents in the microampere range,the cost and delicacy of such relays as compared to a small neon lampwould make it practically impossible to use.

The invention generally provides at each master station a source ofelectric current connected through a resistance network to the stationindicating neon lamp. The source may be A.C., in which case rectifierand filter means (as a capacitor) are used. If DC. is used, then norectifier or filter are needed. Direct current connections from thenetwork are provided to the station selector switch. Each master stationis provided with similar hookups. When one calling master station hasits station selector switch provide a communication channel to anycalled station (either master or remote) so that these two stations arecommunicating with each other, each resistance network at each of suchbusy stations (if both are masters) is unaffected. This will result inthe sta- 3,142,725 Patented July 28, 1964 tion neon light remaining inits normal condition (either on or 01f as desired). If the normalcondition of such a neon light at a station is on, it can function alsoas a pilot light. Assume now that a calling master station iscommunicating with a called station (either master or remote) and athird station (master) is attempting to call either of these two busystations. The third station (master) will have its station selectorswitch set so that the third station is a calling station and ismetallically connected to another calling station. The inventionprovides that when two calling stations are connected, the resistancenetwork at the third station is unbalanced and the neon light of thatthird station goes to an off-normal condition. This is true whether thethird station (master) was calling the first or second. station.

In order that the invention may be understood, it will now be disclosedin connection with the accompanying drawings wherein:

FIGURE 1 shows an exemplary system embodying the invention having threemaster stations and four remote stations.

FIGURE 2 shows a modification of the network and lamp wiring which maybe used instead of the resistance networks and lamp wiring illustratedin FIGURE 1.

FIGURE 3 is a further modification showing the station wiring.

Referring first to FIGURE 1, three master stations A, B and C areillustrated. Since the wiring in each station is the same, it isnecessary only to describe in detail the wiring at one station.Referring for example to station A, wires 10 and 11 are line wiresconnected to a suitable source of electric power such as, for example, aconventional volt, 60 cycle line. In the system as illustrated, it isnecessary that each station be connected in a similar manner withrespect to polarity to the power line. Accordingly, wire 11 will beregarded as the common or so-called ground. Wire N has ON-OFF switch 12connected thereto, the other terminal of this switch going to resistor14. The circuit continues through resistor 14 to half wave rectifier 16,then on to junction point 17. Between junction point 17 and wire 11there is connected filter capacitor 18. From wire 11 there is connectedcapacitor 21 shunted by resistor 22 going to bus wire 24. Junction point17 is connected to one terminal of resistor 25, the other terminal ofwhich is connected to junction point 26. Between junction point 26 andwire 24 is connected neon light 27. This neon light may be of the typeavailable in the market as pilot lights, whose characteristics have beenpreviously pointed out. Resistor 25 is quite a bit larger in value thanresistor 22 and both of them together function as ballasts for lamp 27.As an example, for a conventional 115 volt line, resistor 25 may have avalue of about one-third megohm while resistor 22 can be about .1megohm.

Junction point 26 is connected to one terminal of each of resistors 29and 30. The remaining terminal of resistor 29 is connected to switchpoint 31 forming part of Talk-Listen switch TLA. Talk-Listen switch TLAhas seven movable contacts 32 to 38 inclusive.

7 The seven movable contacts are all mechanically coupled together andcan have one of three positions. The position illustrated in thedrawings is the normal Standby position. The remaining two positions arerespectively Listen and Talk. Thus for example, movable contact 32cooperates respectively with fixed contacts 328, 32L and 32T. Theremaining movable contacts cooperate with correspondingly numbered fixedcontacts. In practice, the Talk-Listen switch may be selectively movedto Standby, Listen or Talk position. Fixed contacts 32T and 358 areconnected by wires to junction point 40. Junction point 40 is connectedto one terminal of center tapped winding 4-1 of audio frequency outputtransformer 42. Winding 41 has its lower terminal connected to junctionpoint 43 which in turn is connected to fixed contacts 338T and 368.Secondary winding 41 of transformer 42 has its center connected throughresistor 45 to moveable contact 34 of the Talk-Listen switch. Resistor45 will usually be small and can be reduced to zero value. Contact 345is dead and contacts 34L and 34T are connected together to bus wire 24.Bus wire 24 is also connected to the center tap of winding 47 of audiofrequency input transformer 48. Winding 47, which is the primary, hasone terminal which is respectively connected to contacts 351 and 32L.Winding 47 has its bottom terminal connected respectively to fixedcontacts 36T and 33L. It will be noted that fixed contacts 355 and SSLare connected together and that fixed contacts 365 and 36L are alsoconnected together. Movable contacts 35 and 36 are connected to the twoterminals of the winding of transducer 49 which can function either as aspeaker or as a transmitter. A simple form of such transducer is aconventional electromagnetic type speaker.

Transformer 48 has secondary winding t whose terminals are connected tothe input terminals of a conventional audio frequency amplifier 51. Theoutput of amplifier 51 is connected through channel 52 to primarywinding 53 of output transformer 42..

No attempt is made to show the circuit details between the secondary oftransformer 48 and the primary of transformer 42. It is necessary thatsome amplification be provided for voice currents fed into primarywinding 47 of transformer 48 to provide amplified voice currents insecondary winding 41 of transformer 42. It is therefore understood thatthe showing with regard to the input and output of amplifier 51 isdiagrammatic.

Station A has voice channel input terminals 55 and 56 connectedrespectively to fixed contacts 37T and 38T. Fixed contacts 37T and 38Tare respectively connected by jumpers to fixed contacts 358 and 36S.Terminals 55 and 56 of station A are the master input terminals MI andare connected by wires 57 and 58 respectively to a pair of wires ALI andAL2 forming part of a cable running between the various stations. As iswell known in intercom systems, each station requires its own pair ofwires as a communication input channel connecting all other stationswhich may want to call.

Station A is provided with station selector switch SSA having as manybuttons connected to communication channels as there are stations to becalled. For example, station selector switch assembly SSA has switchportion SSAC which can be used to select station C with A as the callingstation. Similarly, switch portion SSAB will be used for selectingstation B. Switch portion SSAR4 will be used to select remote stationR4. Switch portion SSARl is connected to select remote station R1.Similarly, station B has its station selector switch SSB provided withcorresponding switch portions, one of which is SSBR2 connected to remotestation R2 and another is switch portion SSBR4 connected to remotestation R4. Station C has its station selector switch SSC provided withswitch portions SSCR3 and SSCR4 going to remote stations R3 and R4respectively.

Referring back to station A, station selector switch SSA has each switchportion movable to the right for the purpose of selecting a station. Forexample, when switch portion SSARl is moved to the right, movablecontacts 60 and 61 are moved to the right to bridge the respectivecontact pairs. Thus, fixed contact 62 is connected to one terminal ofthe line going to remote station R1. Similarly, fixed contact 63 isconnected to the other terminal of R1. When movable contacts 60 and 61are both moved to the right' (they move simultaneously), contact 62 willbe connected to fixed contact 64 while contact 63 will be connected tofixed contact 65. Top right contact 64 and the corresponding contacts inthe other switch portions are connected to wire 63 going to movablecontact 32 of Talk-Listen switch TLA. The bottom right contact 65 andthe other corresponding contacts for the remaining switch portions ofselector switch SSA are all connected to wire 70 going to fixed contact33 of Talk-Listen switch TLA. Wires 68 and 70 are also connected tofixed contacts 37 and 38 respectively of the Talk-Listen switch.

Remaining stations B and C are correspondingly wired as illustrated inFIGURE 1. Assuming that the various stations are energized insofar asinput lines 10 and 11 are concerned, and assuming each station is inStandby condition, each neon light in each station will be on. Referringfor example to station A, current will flow from wire 10 through closedswitch 12 to junction 26 through lamp 27, thence through resistor 22 towire 11. With the Talk-Listen switch in the Standby position as shown,and with station selector switch SSA as shown, there will be no otherpath for electric current except the one traced above. In fact, it makesno difference what the condition of Talk-Listen switch TLA is, the samesituation will prevail so long as all switch sections of stationselector switch SSA are open as shown.

Now let it be assumed that station A wants to call station B. At stationA, selector switch portion SSAB is operated. This will connect station Ato station B via line Wires BLll and BLZ. Assuming that station B is notbusy and has all its sections of its selector switch $88 in the openposition and its Talk-Listen switch TLB in the Standby position, thepotential at junction 26 of station A can appear at terminals 55 and 56of station E. There will be no current path in station B from terminals55 and 56 on the one hand and line 11 in station B on the other hand.Hence for all intents and purposes, lamp 27 will be in the same circuitas before and will continue to be on. At the same time, thecorresponding lamp at station B will also be on. This will indicate tostation A that station B is not busy.

When station A first selects station B by operating selector switchstation SSAB, Talk-Listen switch TLA at station A will be at Standby. Assoon as A sees that B is not busy, he moves his Talk-Listen switch TLAto Talk position. B will hear A at full volume. B will be at Standbyposition. Voice currents coming in at terminals 55 and 56 at B will befed through switch contacts 35 and 36 at station B to speaker 49 atstation B. As soon as B operates selector switch section SSBA to connectto A and moves Talk-Listen switch TLB (not shown) to Listen, theresistor networks for the neon lights at each station will beindependently energized. Thus the neon lights will remain on as long asA and B communicate with each other. Switch TLB corresponds to switchTLA but is for station B. The same change is made for station C.

Now assume that C wants to talk to A who is busy with B. C firstoperates station selector switch SSCA while keeping T alk-Listen switchTLC (not shown) at Standby. With A busy, an alternative shunt circuitaround neon light 27 for station C will be provided. This circuit may betraced as follows.

Beginning with station C, junction point 26, current can flow inparallel through resistors 29 and 30 in station C through wires 68 and70 to selector switch portion SSCA, thence line wires ALI and AL2 toterminals 55 and 56 of station A. From terminals 55 and 56, current canflow through switch contacts 355 and 36S and along the 7 wires tojunction points 40 and 43. It will be noted that the condition ofTalk-Listen switch TLA at station A is not important so long as it isnot in Standby condition. From junction points 40 and 43, current willflow through winding 41 of transformer 42 through the center tap tomovable contact 34 of station A. From movable contact 34 current cancontinue through switch contacts 34L or 314T at station A to wire 24,thence through resistor 22 to wire 11. The resistance of this shunt pathis low enough so that the potential between point 26 and wire 24 atstation C drops due to the increased potential drop across resistor 25at station C. The result is that neon light 27 at station C will go out.

In the assumed situation above where station A and station B are bothinterconected and are both master stations, a similar shunt path can betracedthrough station B. If station A were connected with a remotestation, such as for example R4, station C would get a busy signal iftrying to select R4. This would happen for the reason that station Cwould be connected to station A through lines R4L1 and R4L2. Thus inreality, the condition of station A would be responsible for station Chaving the light go out when trying to get R4. It is of course possiblefor station C to proceed in spite of the busy signal and become a partyto the A and B hookup.

Referring now to FIGURE 2', it is possible to reverse the action of theneon lights at any one or all stations. Thus in the system previouslydescribed, the neon light is normally on during Standby and during use.When a station is trying 'to call another station and finds it busy, thebusy condition will be indicated by the light going out at the stationtrying to make a call. The modification illustrated in FIGURE 2 changesthis arrangement so that the light is normally out but goes on to showthat another station being called is busy. This modification is providedby the simple expedient of moving neon light 29 from the positionillustrated in FIGURE 1 to the position illustrated in FIGURE 2. In thismodified position, the neon light is connected in series betweenjunction 26 and the adjacent terminal of resistor 25. Everything elsewill remain the same.

Referring now to FIGURE 3, a still further modification is shown whereinan isolation transformer is provided from line wires and 11 and theremainder of the system. Isolation transformer 80 has primary winding 81connected to supply wires 10 and 11 when switch 12 is closed. Primarywinding 81 has the bottom terminal connected to wire 11 and alsoconnected to conductor 24' through isolation resistor 83. Conductor 24is equivalent to conductor 24 in FIGURES 1 and 2. The remainder of thecircuit beyond lamp 27 in FIGURE 3 is the same as in FIGURE 1.Transformer 80 has secondary 84 connected in exactly the same fashion asin FIGURE 1. In this modified form illustrated in FIGURE 3, resistor 14of FIGURES 1 and 2 may be provided ahead of rectifier 16 in FIGURE 3 ornot. Resistor 14 limits surge current through rectifier 16 if suchprotection is desired. Instead, secondary winding 84 may serve to limitthe amount of surge current drawn from wires 10 and 11.

In FIGURE 3, resistor 83 may be considered as equivalent to resistor 22of FIGURES 1 and 2. The operation of a system utilizing thismodification illustrated in FIG- URE 3 is the same as described inconnection with FIG- URE l. Resistor 83 provides the equivalent returnpath to common power line 11 as provided by resistor 22 in FIGURE 1.Hence when a busy circuit is selected, the DC. return path will besimilar to that described in connection with FIGURE 1. Stations usingthe arrangement of FIGURE 3 must be connected with proper polarity tothe power line.

An intercom system may have station A wired as illustrated in FIGURE 1,station B wired as illustrated in FIGURE 2 and station C wired asillustrated in FIGURE 3. Any combination of modifications may be used.

In connection with resistors 29 and 30, it will be noted that these tworesistors at a calling station are connected across secondary 41 of theamplifier output transformer at any called station. Because of this, itis desirable to have the values of resistors 29 and 30 (they should beequal) high enough so that secondary 41 of audio frequency transformer42 at any called station will not be shorted. Secondary 41 of thetransformer will have a low ohmic resistance for direct current. Insofaras re sistors 29 and 30 are concerned, each of these may have a value ofsubstantially less than the value of resistor 22. Thus in the assumedexample, details of which have been given before, resistor 22 may have avalue of about 100,- 000 ohms and resistors 29* and 30* each may have avalue of between about 30,000 and 40,000 ohms, although this value maybe substantially greater or less as desired. The value of resistor 455in the center tap connection can vary anywhere from zero up to about thesame as each of resistors 29 or 30. In an actual example of such asystem, filter capacitor 18 may be quite large and have a value of about20 mfd'. Capacitor 21 should be large enough to bypass audio frequenciesand can have a value of about .05 microfarad. The remaining componentssuch as transformers 42 and 48 will be conventional in the intercom art.

A master station embodying the present invention will have a ballastresistance network for a gas lamp energized from a suitable source ofdirect current. This ballast resistance network will also have resistorsconnecting a point on the ballast resistance network through a Talk-Listen switch in a Standby position to the line wires going from thecalling station through a station selector switch to the appropriateline wires of a called station. This master called station will have itsinput line connected at all times across the secondary winding of itsaudio frequency output transformer. The center tap of this particularsecondary output winding will be connected to bus wire 24 through aTalk-Listen switch only when the Talk-Listen switch is away from theStandby position. When such a master calling station is connected to amaster called station by virtue of the operation of a station selectorswitch, and if the master called station is talking or listening, thanthe busy called station will provide a path in shunt to a portion of thegas tube normal resistance network. This shunt path will disturb thenormal potential relationships which are present in the ballast networkat the calling master station when such calling master station is notconnected to any other station or is connected to some other stationwhich is not busy. It is understood that this also applies when aconnection is established to a remote station which happens to be busywith another master station. This is true, as has been previouslypointed out, because of the hookup of the remote station to a masterstation.

A system embodying the present invention is highly advantageous insofaras reducing the number of interconnecting wires between stations isconcerned. Between any two stations which are in communication with eachother, there is provided a voice current channel (this consisting of apair of wires between stations), this voice current channel alsoproviding one-half of a signal channel for indicating a busy signal.Since the busy signal currents go the same direction in the two wiresmaking a voice channel, it is possible to arrange for these currents ata station to cancel out insofar as having any undesired effects in thevoice communication channel is concerned. For signal purposes, however,the two wires making up a voice current channel cooperate with thecommon return of a power line energizing system for providing a signalchannel. It is understood that every master station in a systemembodying the present invention must be correctly connected to a commonpower energizing system so that the power line itself can function aspart of the signal channel.

What is claimed is:

1. In an intercom system having a plurality of master stationsinterconnected by individual pairs of wires providing voice channels andhaving a common interstation ground separate from the voice channels,each master station having: an audio frequency amplifier having anoutput audio frequency transformer with its secondary winding connectedat all times to its own voice channel input; a talk-listen switch havingstand-by, talk and listen positions; station selector switch means forselecting an outgoing line to a called station; a signalling systemeffective only on stand-by and, after selecting another station to becalled, to determine if the called station is busy, said signallingsystem including; two equal load resistors connected only on stand-bythrough its talk-listen switch and through its station selector switchto a selected outgoing voice channel so that said two load resistors inseries are connected across the secondary winding of the output audiofrequency transformer of another master station;

a source of direct potential connected between a high potential terminaland the common interstation ground; a gas tube having a breakdownpotential lower than the direct potential available at the station; atwo-part ballast resistor network for said gas tube; one such ballastresistor having one terminal connected to the high potential terminal ofthe voltage supply source; the other ballast resistor being smaller thanthe first one and having one terminal thereof connected to theinterstation ground; a connection from the other terminal of the secondballast resistor to the center of the output transformer secondarywinding through its talk-listen switch only when said talk-listen switchis otf stand-by; and circuit connections involving the other terminalsof the ballast resistors, the gas tube terminals and the common terminalof the load resistors and completed by its talk-listen switch at standbyonly and the talk-listen switch at a connected different master stationfor determining whether the gas tube at a calling master station is onor off; said two conditions being determined by whether the talk-listenswitch at a connected master station is on stand-by or oif stand-by;said gas tube condition indicating busy or idle status of any calledstation.

2. The system according to claim 1 wherein the connection between thesecondary winding center tap and the other terminal of the secondballast resistor includes a resistor whose value is smaller than any ofthe previously identified resistors.

3. The system according to claim 1 wherein said gas tube is connectedbetween the other terminals of the two ballast resistors and whereinsaid other terminal of the first ballast resistor is connected to thejunction of the two load resistors.

4. The system according to claim 1 wherein said gas tube is connectedbetween the other terminal of the first ballast resistor and the commonterminal of the two load resistors and wherein the other terminal of thesecond ballast resistor is connected through the talk-listen switch tothe center tap of the output secondary winding only when the talk-listenswitch is off stand-by.

References Cited in the tile of this patent UNITED STATES PATENTS2,390,203 Campbell Dec. 4, 194-5

1. IN AN INTERCOM SYSTEM HAVING A PLURALITY OF MASTER STATIONSINTERCONNECTED BY INDIVIDUAL PAIRS OF WIRES PROVIDING VOICE CHANNELS ANDHAVING A COMMON INTERSTATION GROUND SEPARATE FROM THE VOICE CHANNELS,EACH MASTER STATION HAVING: AN AUDIO FREQUENCY AMPLIFIER HAVING ANOUTPUT AUDIO FREQUENCY TRANSFORMER WITH ITS SECONDARY WINDING CONNECTEDAT ALL TIMES TO ITS OWN VOICE CHANNEL INPUT; A TALK-LISTEN SWITCH HAVINGSTAND-BY, TALK AND LISTEN POSITIONS; STATION SELECTOR SWITCH MEANS FORSELECTING AN OUTGOING LINE TO A CALLED STATION; A SIGNALLING SYSTEMEFFECTIVE ONLY ON STAND-BY ND, AFTER SELECTING ANOTHER STATION TO BECALLED, TO DETERMINE IF THE CALLED STATION IS BUSY, SAID SIGNALLINGSYSTEM INCLUDING; TWO EQUAL LOAD RESISTORS CONNECTED ONLY ON STAND-BYTHROUGH ITS TALK-LISTEN SWITCH AND THROUGH ITS STATION SELECTOR SWITCHTO A SELECTED OUTGOING VOICE CHANNEL SO THAT SAID TWO LOAD RESISTORS INSERIES ARE CONNECTED ACROSS THE SECONDARY WINDING OF THE OUTPUT AUDIOFREQUENCY TRANSFORMER OF ANOTHER MASTER STATION; A SOURCE OF DIRECTPOTENTIAL CONNECTED BETWEEN A HIGH POTENTIAL TERMINAL AND THE COMMONINTERSTATION GROUND; A GAS TUBE HAVING A BREAKDOWN POTENTIAL LOWER THANTHE DIRECT POTENTIAL AVAILABLE AT THE STATION; A TWO-PART BALLASTRESISTOR NETWORK FOR SAID GAS TUBE; ONE SUCH BALLAST RESISTOR HAVING ONETERMINAL CONNECTED TO THE HIGH POTENTIAL TERMINAL OF THE VOLTAGE SUPPLYSOURCE; THE OTHER BALLAST RESISTOR BEING SMALLER THAN THE FIRST ONE ANDHAVING ONE TERMINAL THEREOF CONNECTED TO THE INTERSTATION GROUND; ACONNECTION FROM THE OTHER TERMINAL OF THE SECOND BALLAST RESISTOR TO THECENTER OF THE OUTPUT TRANSFORMER SECONDARY WINDING THROUGH ITSTALK-LISTEN SWITCH ONLY WHEN SAID TALK-LISTEN SWITCH IS OFF STAND-BY;AND CIRCUIT CONNECTIONS INVOLVING THE OTHER TERMINALS OF THE BALLASTRESISTORS, THE GAS TUBE TERMINALS AND THE COMMON TERMINAL OF THE LOADRESISTORS AND COMPLETED BY ITS TALK-LISTEN SWITCH AT STANDBY ONLY ANDTHE TALK-LISTEN SWITCH AT A CONNECTED DIFFERENT MASTER STATION FORDETERMINING WHETHER THE GAS TUBE AT A CALLING MASTER STATION IS ON OROFF; SAID TWO CONDITIONS BEING DETERMINED BY WHETHER THE TALK-LISTENSWITCH AT A CONNECTED MASTER STATION IS ON STAND-BY OR OFF STAND-BY;SAID GAS TUBE CONDITION INDICATING BUSY OR IDLE STATUS OF ANY CALLEDSTATION.